924 



DR F. R. C. REED ON THE 



(2) 



(3) 



(4) 



9-2 



12'3 



12-2 mm. 



10'0 



13-8 



12*5 mm. 



5'2 



8'0 



9'4 mm. 



width of shell, shallow, rounded, with floor more or less produced into tongue ; lateral 

 margins of sinus angular, more or less strongly elevated on front margin ; floor of 

 sinus with single narrow median rib of variable length and sometimes nearly 

 obsolete ; lateral portions of valve gently convex, with traces of one or more angular 

 short marginal plications, often obsolete, forming zigzag junctions of margin ; beak 

 small, acute, elevated, very slightly incurved, higher than that of opposite valve ; 

 triangular delthyrium below beak. Brachial valve usually more convex than 

 pedicle-valve, with a more or less developed fold towards anterior margin composed 

 of a pair of rather closely placed small angular or subangular plications of variable 

 length, becoming highest on anterior margin ; lateral portions of valve more or less 

 convex and dependent, occasionally with weak marginal plications ; beak obtuse, 

 low, small. Surface of valves smooth. 



Interior of pedicle-valve with median septum fully one-third the length of the 

 valve. Interior of brachial valve with shorter median septum. 



Dimensions. — 



(1) Type. 

 Length . . . 10 "5 



Width . . . 10-5 



Thickness . . .6*8 



Horizon. — Stinchar Limestone Group. 

 Locality. — Craighead. 



Remarks. — This species varies to a considerable extent in certain directions, but 

 it always retains the double plication on the fold, and never has more than a single 

 plication in the sinus. The width relative to the length, the degree of inflation and 

 convexity of the valves, the prominence and length of the plications on the fold and 

 of the one in the sinus, and the number and development of the lateral marginal 

 plications are variable. The forms from Craighead with more accentuated plications 

 and greater anterior width Davidson referred to Rh. semula, Salter,* which comes 

 typically from the Chair of Kildare. The shells from the Balclatchie conglomerate 

 figured by Davidson t as Rh. balclatchiensis differ in their greater length, more 

 elongated shape, and obsolete median plication in the sinus, and constitute a fairly 

 well-marked variety (see next page). The specimen from Minuntion J attributed by 

 him to this species is completely distinct, and is described in this memoir as Triplecia 

 craigensis, sp. nov. (see p. 908). 



The small specimen from Penkill attributed to Rh. semula by Davidson § and 

 figured as such is too poorly preserved for satisfactory identification, and the figure 

 is incorrect. 



C. balclatchiensis is undoubtedly closely allied to C. Thomsoni (Davidson), as 



* Davidson, op. cit., vol. lii, p. 188, pi. xxiv, fig. 21.' 



t Davidson, op. cit., vol. v, Silur. Supjrt., pi. x, fig. 16. 



} Davidson, op. cit., vol. v, Silur. Suppl, p. 200, pi. xi, fig. 23. 



§ Davidson, op. cit, vol. v, Silur. Suppl., v, pi. x, fig. 17. 



